Silicon photonics multicarrier optical transceiver

ABSTRACT

Disclosed herein are techniques, methods, structures and apparatus that provide a silicon photonics multicarrier optical transceiver wherein both the transmitter and receiver are integrated on a single silicon chip and which generates a plurality of carriers through the effect of an on-chip modulator, amplifies the optical power of the carriers through the effect of an off-chip amplifier, and generates M orthogonal sets of carriers through the effect of an on-chip basis former.

RELATED APPLICATIONS

This Application is a continuation claiming the benefit under 35 U.S.C.§ 120 of U.S. application Ser. No. 15/971,889, filed on May 4, 2018, andentitled “SILICON PHOTONICS MULTICARRIER OPTICAL TRANSCEIVER,” which ishereby incorporated herein by reference in its entirety

U.S. application Ser. No. 15/971,889 is a continuation claiming thebenefit under 35 U.S.C. § 120 of U.S. application Ser. No. 13/894,367,filed on May 14, 2013, and entitled “SILICON PHOTONICS MULTICARRIEROPTICAL TRANSCEIVER”, which is hereby incorporated herein by referencein its entirety,

U.S. application Ser. No. 13/894,367 claims the benefit under 35 U.S.C.§ 119(e) of U.S. Provisional Application Ser. No. 61/646,517, filed onMay 14, 2012, entitled “SILICON PHOTONICS MULTICARRIER OPTICALTRANSCEIVER,” which is hereby incorporated herein by reference in itsentirety.

TECHNICAL FIELD

This disclosure relates generally to the field of optical communicationsand in particular to techniques, methods and apparatus pertaining tosilicon photonics multicarrier coherent transceiver that allows forhigher data rates without requiring higher speed analog-to-digitalconverters (ADCs) and digital-to-analog converters (DACs) or higherorder constellations.

BACKGROUND

Contemporary optical communications and other systems require reliabletransceivers exhibiting high data rates. Consequently, methods,structures or techniques that facilitate the development or improvementof such transceivers—particularly those that do not require higher speedADCs or DACs—would represent a welcome addition to the art.

BRIEF SUMMARY

An advance in the art is made according to an aspect of the presentdisclosure directed to a silicon photonics multicarrier coherenttransceiver wherein both receiver and transmitter are integrated ontothe same silicon substrate.

Viewed from a first aspect, the present disclosure is directed to asilicon photonics multicarrier coherent transceiver that is integratedonto a single silicon substrate and employs a single laser—that isadvantageously either off-chip or integrated on-chip using a gain chip.Advantageously transceivers according to the present disclosure generatea plurality of carriers using on-chip modulators, employ off-chipamplifiers to boost optical power, and uses basis former(s) to generateM orthogonal sets of carriers. Of particular advantage is the use of abasis former that includes a power splitter connected to M waveguides ofdifferent length.

BRIEF DESCRIPTION OF DRAWINGS

A more complete understanding of the present disclosure may be realizedby reference to the accompanying drawings in which:

FIG. 1 shows a schematic top-view of a multi-carrier coherenttransceiver photonics integrated circuit according to an aspect of thepresent disclosure;

FIGS. 2A and 2B show a schematic of basis formers according to an aspectof the present disclosure wherein FIG. 2A depicts a demultiplexer eachoutput having one carrier while FIG. 2B includes M arms of differentpath lengths connected to a power splitter, each output having all threecarriers but with relative phase shifts of ±120° between them such thatthe three sets are mutually orthogonal;

FIG. 3 shows another schematic configuration of an alternativemulti-carrier coherent transceiver photonics integrated circuitaccording to an aspect of the present disclosure; and

FIG. 4 shows another schematic configuration of an alternativemulti-carrier coherent transceiver photonics integrated circuitaccording to an aspect of the present disclosure.

DETAILED DESCRIPTION

The following merely illustrates the principles of the disclosure. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the disclosure and are includedwithin its spirit and scope.

Furthermore, all examples and conditional language recited herein areprincipally intended expressly to be only for pedagogical purposes toaid the reader in understanding the principles of the disclosure and theconcepts contributed by the inventor(s) to furthering the art, and areto be construed as being without limitation to such specifically recitedexamples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosure, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently-known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the invention.

In addition, it will be appreciated by those skilled in art that anyflow charts, flow diagrams, state transition diagrams, pseudocode, andthe like represent various processes which may be substantiallyrepresented in computer readable medium and so executed by a computer orprocessor, whether or not such computer or processor is explicitlyshown.

In the claims hereof any element expressed as a means for performing aspecified function is intended to encompass any way of performing thatfunction including, for example, a) a combination of circuit elementswhich performs that function or b) software in any form, including,therefore, firmware, microcode or the like, combined with appropriatecircuitry for executing that software to perform the function. Theinvention as defined by such claims resides in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the claims call for. Applicant thusregards any means which can provide those functionalities as equivalentas those shown herein. Finally, and unless otherwise explicitlyspecified herein, the drawings are not drawn to scale.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the disclosure.

By way of some additional background, it is noted that previousdemonstrations of multicarrier coherent transceivers have been made. Forexample, R. Nagarajan has described in a paper entitled “10 Chanel, 100Gbit/s per Channel, Dual Polarization, Coherent QPSK, Monolithic InPReceiver Photonic Integrated Circuit” which was presented at the OpticalFiber Communication Conference, 2011, p. OML7, however devices such asthose described therein include 10 integrated lasers. Those skilled inthe art will quickly recognize the difficulty associated with makingsuch devices with high yield on a single chip and in particular wherethey all must be wavelength stabilized. Additionally, the devicesdisclosed therein use InP as the integration platform which is difficultto fabricate in high yield. Furthermore, the devices disclosed thereinuse separate chips for transmit and receive which requires additionalpackaging. Finally, devices such as those disclosed by Nagarajan mayexhibit a poor spectral efficiency as they are limited by theirwavelength stability to a relatively wide wavelength separation and suchdevices may have channels that are relatively narrow-band and thussusceptible to nonlinearities.

Turning now to FIG. 1, there is shown a schematic top-view of amulticarrier coherent transceiver PIC according to an aspect of thepresent disclosure. As depicted in FIG. 1, N=M=3; PBSR=polarizationsplitter and rotator; ITLA—integrated tunable laser assembly;PM-EDFA-polarization maintaining Er-doped fiber amplifier; andPD=photodetector.

Operationally, an integrated tunable laser assembly (ITLA) generates anarrow-linewidth continuous-wave laser (optical) signal. The generatedoptical signal enters a silicon PIC (SiPhPIC) where it is directed to aphase modulator which is preferably driven sinusodially at a frequencyf. As may be appreciated, a single phase modulator may readily generatetwo sidebands exhibiting the same height as the carrier, resulting inthree carriers. As may be further appreciated, the phase modulator maybe replaced or substituted by an amplitude modulator or a combination ofan amplitude modulator and a phase modulator.

Advantageously, other modulator schemes known in the art may be used togenerate multiple carriers. More particularly, and by way of example, amodulator positioned within a ring resonator may be used to generatemultiple carriers. Yet another exemplary alternative may include anarrangement of single-sideband modulators.

For the purposes of this discussion and as depicted in FIG. 1, we let Nbe the total number of carriers exiting the modulator assembly.Typically, these N carriers do not exhibit enough optical power tosupply both the modulator inputs in the transmitter and the localoscillators in the receiver. As a result—and according to an aspect ofthe present disclosure—the N carriers are directed from the PIC to anoff-chip, Er-doped fiber amplifier (EDFA) where they are amplifiedthrough the effect of the EDFA.

As may be appreciated, it is preferred that the EDFA be polarizationmaintaining, however this is not absolutely necessary. Alternatively, asemiconductor gain element known in the art may be employed wherein thatgain element is attached directly to the PIC or coupled via fibercoupler.

The amplified carriers are then directed from the EDFA to the PIC wherethey enter a “basis former”. The basis former splits the N carriers intoM orthogonal sets, where M<=N.

Two examples of basis formers according to an aspect of the presentdisclosure for three carriers (N=3) are shown schematically in FIGS.2A-2B. As depicted in FIG. 2A, the basis former comprises a wavelengthdemultiplexer wherein each output is a separate one of the threecarriers. Such a basis former may be constructed from an arrayedwaveguide grating, Mach-Zehnder filters, or ring resonator filters. FIG.2B shows an alternative basis former according to an aspect of thepresent disclosure. More specifically, the alternative embodiment shownin FIG. 2B comprises a 1×M power splitter followed by M waveguides oflinearly increasing path length. In an exemplary embodiment, the pathlength difference between successive waveguides is defined byc/(fn_(g)M), where c is the speed of light in a vacuum and n_(g) is thewaveguide group index.

One appreciable advantage of this configuration shown in FIG. 2B is thatit does not require filters to remain constant despite temperaturechanges, as is the case with the configuration shown in FIG. 2A.Additionally, data for each “channel” is spread over the entire signalspectrum, thereby reducing effects of nonlinearities.

With continued reference to FIG. 1, it may be observed that outputs ofthe basis former are directed to a number of quadrature phase-shiftkeying (QPSK) modulators. As depicted therein, the number of QPSKmodulators in 2N. The outputs of the modulators are subsequentlycombined by power converter(s). One half of the modulated outputs arepolarization rotated, and then further combined through the effect ofpolarization splitter and rotator and subsequently output as combined Txoutput signal.

Notably, modulators other than the QPSK modulators shown arecontemplated according to the present disclosure. More particularly,8-quadrature amplitude modulation (QAM) modulators may be employedequally well. One constraint on the modulator(s) employed however isthat the modulator symbol rate should equal to or be less than f (asdefined above).

Advantageously, the polarization splitter and rotator (PBSK) maycomprise a 2D grating coupler for example. When such a 2D gratingcoupler is employed, explicit polarization rotators are not required andinstead the 2D grating coupler combines the co-polarized inputs into asingle, polarization multiplexed output.

With respect to receiver function(s), received signals (Rx input) aresplit through the effect of a PBSR and the resulting portions aredirected into 90 degree hybrids where they are combined with some of theoutputs from the basis former which act as local oscillators. Opticalsignals output from the hybrids are detected by photodetectors anddirected to 4N analog to digital converters for subsequent processing.

Turning now to FIG. 3, there it shows an alternative embodiment of a PICaccording to an aspect of the present disclosure. As shown in FIG. 3, anumber of additional basis formers are employed. Using the example ofN=3 carriers, each of the individual basis formers outputs the Ncarriers which are then applied to an individual QPSK modulator or 90degree hybrid as shown. A particular advantage of this exemplaryconfiguration is that there are fewer waveguide crossings on the PIC ascompared with the PIC shown in FIG. 1.

Turning now to FIG. 4, there is shown yet another PIC according to anaspect of the present disclosure. More specifically, the PIC shown inFIG. 4 includes an integrated CW laser.

Those skilled in the art will readily appreciate that while the methods,techniques and structures according to the present disclosure have beendescribed with respect to particular implementations and/or embodiments,those skilled in the art will recognize that the disclosure is not solimited. More particularly, the variations depicted in the FIGUREs maybe combined as appropriate. For example, the integrated laser of FIG. 4may be included in the arrangement of FIG. 3 or FIG. 1. Accordingly, thescope of the disclosure should only be limited by the claims appendedhereto.

What is claimed is:
 1. An optical transceiver comprising: a siliconsubstrate; first and second transmitter modulators integrated on thesilicon substrate; first and second photodetectors integrated on thesilicon substrate; a carrier generator configured to receive as input anoptical signal having an input carrier from an optical signal source andto generate, from the input carrier, a plurality of output carriersincluding a first output carrier and a second output carrier; a firstpower splitter configured to provide the first output carrier to thefirst transmitter modulator and to the first photodetector; and a secondpower splitter configured to provide the second output carrier to thesecond transmitter modulator and to the second photodetector.
 2. Theoptical transceiver of claim 1, further comprising a wavelengthdemultiplexer configured to provide the first output carrier to thefirst power splitter and the second output carrier to the second beamsplitter.
 3. The optical transceiver of claim 2, wherein the wavelengthdemultiplexer is coupled to the carrier generator through an opticalamplifier disposed outside the silicon substrate.
 4. The opticaltransceiver of claim 1, further comprising: a third power splitter; andfirst and second waveguides having different lengths, wherein the firstwaveguide couples the third power splitter to the first power splitterand the second waveguide couples the third power splitter to the secondpower splitter.
 5. The optical transceiver of claim 4, furthercomprising a third waveguide coupled to the third power splitter,wherein the first waveguide has a first length, the second waveguide hasa second length and the third waveguide has a third length, wherein thefirst, second and third lengths substantially conform to a linearlyincreasing relationship.
 6. The optical transceiver of claim 1, whereinthe carrier generator is integrated on the silicon substrate.
 7. Theoptical transceiver of claim 1, wherein the carrier generator comprisesa signal modulator.
 8. The optical transceiver of claim 1, wherein thefirst photodetector is configured to beat the first output carrier withan input signal and the second photodetector is configured to beat thesecond output carrier with the input signal.
 9. The optical transceiverof claim 1, wherein the first and/or second transmitter modulatorscomprises a quadrature phase shift key (QPSK) modulator.
 10. The opticaltransceiver of claim 1, further comprising a power combiner coupled tothe first and second transmitter modulators.
 11. An optical transceivercomprising: a silicon substrate; first and second transmitter modulatorsintegrated on the silicon substrate; first and second photodetectorsintegrated on the silicon substrate; a carrier generator configured toreceive as input an optical signal having an input carrier from anoptical signal source and to generate, from the input carrier, aplurality of output carriers including a first output carrier and asecond output carrier; and a power splitter having first and secondoutputs, wherein the first output of the power splitter is configured toprovide the first output carrier to the first transmitter modulator andthe second output carrier to the second transmitter modulator, and thesecond output of the power splitter is configured to provide the firstoutput carrier to the first photodetector and the second output carrierto the second photodetector.
 12. The optical transceiver of claim 11,further comprising: a first wavelength demultiplexer coupled to thefirst output of the power splitter and configured to provide the firstoutput carrier to the first transmitter modulator and the second outputcarrier to the second transmitter modulator.
 13. The optical transceiverof claim 12, further comprising: a second wavelength demultiplexercoupled to the second output of the power splitter and configured toprovide the first output carrier to the first photodetector and thesecond output carrier to the second photodetector.
 14. The opticaltransceiver of claim 11, wherein the power splitter is a first powersplitter, and wherein the optical transceiver further comprises: asecond power splitter coupled to the first output of the first powersplitter; and first and second waveguides having different lengths, thefirst waveguide coupling the second power splitter to the firsttransmitter modulator and the second waveguide coupling the second powersplitter to the second transmitter modulator.
 15. The opticaltransceiver of claim 14, further comprising: a third power splittercoupled to the second output of the first power splitter; and third andfourth waveguides having different lengths, the third waveguide couplingthe third power splitter to the first photodetector and the fourthwaveguide coupling the third power splitter to the second photodetector.16. The optical transceiver of claim 11, wherein the carrier generatoris integrated on the silicon substrate.
 17. The optical transceiver ofclaim 11, wherein the carrier generator comprises a signal modulator.18. The optical transceiver of claim 11, wherein the first photodetectoris configured to beat the first output carrier with an input signal andthe second photodetector is configured to beat the second output carrierwith the input signal.
 19. The optical transceiver of claim 11, whereinthe first and/or second transmitter modulators comprises a quadraturephase shift key (QPSK) modulator.
 20. The optical transceiver of claim11, further comprising a power combiner coupled to the first and secondtransmitter modulators.